Implemented correct behavior for Command.CanExecute on MenuItem.

src/Avalonia.Controls/MenuItem.cs

@@ -93,6 +93,7 @@ namespace Avalonia.Controls
         static MenuItem()
         {
             SelectableMixin.Attach<MenuItem>(IsSelectedProperty);
+            CommandProperty.Changed.Subscribe(CommandChanged);
             FocusableProperty.OverrideDefaultValue<MenuItem>(true);
             IconProperty.Changed.AddClassHandler<MenuItem>(x => x.IconChanged);
             ItemsPanelProperty.OverrideDefaultValue<MenuItem>(DefaultPanel);
@@ -424,6 +425,40 @@ namespace Avalonia.Controls
             }
         }
 
+        /// <summary>
+        /// Called when the <see cref="Command"/> property changes.
+        /// </summary>
+        /// <param name="e">The event args.</param>
+        private static void CommandChanged(AvaloniaPropertyChangedEventArgs e)
+        {
+            if (e.Sender is MenuItem menuItem)
+            {
+                if (e.OldValue is ICommand oldCommand)
+                {
+                    oldCommand.CanExecuteChanged -= menuItem.CanExecuteChanged;
+                }
+
+                if (e.NewValue is ICommand newCommand)
+                {
+                    newCommand.CanExecuteChanged += menuItem.CanExecuteChanged;
+                }
+
+                menuItem.CanExecuteChanged(menuItem, EventArgs.Empty);
+            }
+        }
+
+        /// <summary>
+        /// Called when the <see cref="ICommand.CanExecuteChanged"/> event fires.
+        /// </summary>
+        /// <param name="sender">The event sender.</param>
+        /// <param name="e">The event args.</param>
+        private void CanExecuteChanged(object sender, EventArgs e)
+        {
+            // HACK: Just set the IsEnabled property for the moment. This needs to be changed to
+            // use IsEnabledCore etc. but it will do for now.
+            IsEnabled = Command == null || Command.CanExecute(CommandParameter);
+        }
+
         /// <summary>
         /// Called when the <see cref="Icon"/> property changes.
         /// </summary>

src/Avalonia.Themes.Default/MenuItem.xaml

@@ -133,4 +133,8 @@
   <Style Selector="MenuItem:empty /template/ Path#rightArrow">
     <Setter Property="IsVisible" Value="False"/>
   </Style>
+
+  <Style Selector="MenuItem:disabled">
+    <Setter Property="Opacity" Value="{DynamicResource ThemeDisabledOpacity}"/>
+  </Style>
 </Styles>